1. Field of the Invention
The present invention relates to a molding die apparatus for molding a disc substrate of a disc-shaped recording medium such as an optical disc and magneto-optical disc by injection molding, a method for molding a disc substrate, and a disc-shaped recording medium.
2. Description of the Related Art
Generally, a disc substrate or a basal plate of a disc-shaped recording medium such as an optical disc and magneto-optical disc is molded by injection molding by the use of a molding die.
Such a molding die is required to suppress generation of burrs around the outer circumference of a molded disc substrate, and to give off gas uniformly when a disc substrate is being molded.
The molding die includes a pair of stationary die and movable die which have a cavity for molding a disc substrate, an outer circumference ring for molding an outer circumference end of a disc substrate, a stamper for transcribing predetermined convexo-concave patterns onto a disc substrate, a driving means for moving the movable die toward and away from the stationary die.
There is formed a stationary side mirror-finished surface at the stationary die on which the stamper is to be mounted. On the other hand, there is formed a movable side mirror-finished surface (referred to as a molding surface, hereinafter) at the movable die, which molds a main surface of a disc substrate. And, there is mounted the outer circumference ring on the outer circumference of the molding surface of the movable die such that the outer circumference ring can move toward the cavity substantially in parallel with the moving direction of the movable die.
The stamper is of a disc-shaped configuration having a center hole, and is abutted to the stationary side mirror-finished surface of the stationary die. The stamper has its inner circumference fixed by an inner stamper holder and has its outer circumference fixed by an outer stamper holder, respectively.
The outer circumference ring is substantially of a ring-shaped configuration, and there is formed a molding surface for molding an outer circumference end of a disc substrate on the inner circumference thereof. The outer circumference ring is moved by an elastic body such as a spring, or a moving means using air pressure, oil pressure, etc.
As shown in FIG. 1, when a movable die 107 is moved toward a stationary die 106 and a molding die having the stationary die 106 and movable die 107 is clamped, there is formed a cavity 108 enclosed by a molding surface 126 of the movable die 107, a main surface of a stamper 113 abutted to the stationary die 106, and a molding surface 135 of an outer circumference ring 116, which space is to be filled with resin material.
At the time of clamping (closing) the molding die, the outer circumference ring 116 is moved toward the stationary die 106 with a predetermined pressure. The molding die is configured such that, when the outer circumference of the outer circumference ring 116 is abutted to an outer stamper holder 115, there is secured a clearance gap 120 for evacuating gas in the cavity 108 between the outer circumference ring 116 and the stamper 113. The clearance gap 120 is designed to have a predetermined size, which size can evacuate only gas in the cavity 108.
When injecting molten resin material into the cavity 108, air in the cavity 108 and gas generated from the molten resin material is evacuated from the clearance gap 120. On filling the cavity 108 with molten resin material, the injection of molten resin material is terminated. At the time of injecting molten resin material, only air and gas is evacuated from the clearance gap 120, and molten resin material is prevented from flowing into the clearance gap 120 so as to suppress generation of burrs at a molded disc substrate.
In injecting molten resin material into the cavity 108, in case the injection pressure is larger than the clamping force, the interval between the stationary die 106 and the movable die 107 is caused to be large. On the other hand, the pressure for causing the outer circumference ring 116 to protrude into the cavity 108 is being kept applied. So, the clearance gap 120 will not be expanded, and generation of burrs can be suppressed.
After completing the injection of molten resin material and cooling down the resin material, opening of the molding die is initiated. At this time, the molded disc substrate is moved together with the molding surface 126 of the movable die 107.
Then, the molded disc substrate is separated from the molding surface 126 of the movable die 107 by sleeve-shaped extruding pins (not shown) and ejected. Thus, the whole molding process composed of a series of molding steps is completed.
After opening the molding die, when the molded disc substrate is separated from the molding surface 126 of the movable die 107, it depends on the configuration of the molding die or molding condition whether the whole main surface of the disc substrate is attached to or released from the molding surface 126 of the movable die 107.
In case the whole main surface of the disc substrate is attached to the molding surface 126 of the movable die 107, separation resistance is prone to be large when the disc substrate is ejected. So, for example, an assistant means for supplying air is used to compulsorily separate the disc substrate. However, it is difficult to spray air upon the whole main surface of the disc substrate uniformly. That is, air is undesirably sprayed upon the center portion of the disc substrate partially where air can easily flow, which results in ill-balanced separation of the disc substrate. So, it is required that, after the molding die is opened, the whole main surface of the disc substrate is released uniformly from the molding surface 126 of the movable die 107.
However, when the outer circumference ring 116 is moved such that it further protrudes into the cavity 108 from the molding surface 126 of the movable die 107, in case a pressure caused by the outer circumference ring 116 is not transmitted to the disc substrate, the whole main surface of the disc substrate cannot be released from the molding surface 126 of the movable die 107.
In reality, in case a disc substrate (to be indicated with a reference numeral of 102) is molded using a molding die having the outer circumference ring 116 shown in FIG. 1, a pressure caused by the outer circumference ring 116 cannot be transmitted to the disc substrate sufficiently. So, the outer circumference of the disc substrate is unstably released, and the inner circumference of the disc substrate remains abutted to the molding surface 126 of the movable die 107. Thus, there arises a problem that a separation pattern 122 is generated between the outer circumference and the inner circumference of the disc substrate 102, as shown in FIG. 2. That is, only the outer circumference ring 116 protrudes toward the side of the disc substrate 102, and the inner and middle circumference of the disc substrate 102 remain abutted to the molding surface 126 of the movable die 107, and thus the separation pattern 122 is generated.
Thus molded disc substrate 102 with the separation pattern 122 not only spoils the beauty of a disc but also makes the optical and electrical characteristics thereof unstable. That is, for example, there arise wobbles in recording/reproducing signals.